Silica glass articles have been used as light transmitting mediums, such as optical fibers and various optical elements. In particular, use of the optical fibers having advantages of light weight, small diameter and no induction, has recently been widened in various industrial fields including communication, image transmission and energy transmission. As one of the fields, use of the optical fiber to transmit ultraviolet rays has been expected in the medical and precise processing fields. However, when glass is irradiated with ultraviolet rays, it deteriorates and its transmission loss increases. That is, there arises a problem in that deterioration takes place because of ultraviolet ray irradiation. Since the transmission loss of a silica optical fiber having the silica glass as the core thereof is smaller than that of an optical fiber made of multicomponent type glass, the silica optical fiber is a preferred element to transmit ultraviolet rays. However, the problem of the deterioration which takes place because of ultraviolet ray irradiation remain unsolved.
There may be a case where a light transmission in silica glass is superior to that in air if the wavelength is not longer than 200 nm. The reason for this lies in that dissociation of an oxygen gas takes place because of ultraviolet ray irradiation in air. Therefore, a high transmission can be expected if the deterioration which takes place because of ultraviolet ray irradiation can be reduced in the wavelength region not longer than 200 nm.
It has been considered that the deterioration which takes place because of ultraviolet ray irradiation is mainly attributed to a defect in glass. In the present invention, the "defect in glass" means a broken portion of the glass network structure or a portion of the glass network structure that is stretched due to a distortion of the glass and is apt to break easily. FIG. 4 shows a plurality of examples of reported defects in glass of silica glass. As representative defects in glass, defects related to E' center (.tbd.Si.) and oxygen-deficient type defects (.tbd.Si--Si .tbd.) are exemplified. The above-mentioned defects in glass absorb ultraviolet rays at wavelengths of 163 nm, 215 nm and 245 nm. It has been considered that the foregoing defects in glass occur in a glass synthesized in an atmosphere somewhat lacking in oxygen, or in a glass having a low concentration of OH groups.
As a technique for reducing deterioration due to ultraviolet ray irradiation of silica glass, a technique has been disclosed in JP-A-5-147966 (hereinafter referred to as Document (1)) (The term "JP-A" used herein means an unexamined published Japanese patent application), in which the content of OH groups in a pure silica core is adjusted to from 10 ppm to 1000 ppm, the contents of F (fluorine) is adjusted to from 50 ppm to 5000 ppm and the contents of Cl (chlorine) is adjusted to substantially zero. An optical fiber thus obtained has an excellent initial characteristic of transmitting ultraviolet rays and is capable of reducing deterioration due to ultraviolet ray irradiation because fluorine is contained in a specific amount.
There are several known techniques that are not aimed to improve deterioration due to ultraviolet ray irradiation but are related to an improvement in radiation resistance of a fiber for transmitting visible rays or near infrared rays. For example, JP-A-60-90853 (hereinafter referred to as "Document (2)), has suggested a process in which any one of a glass soot body, a transparent glass preform and an optical fiber is processed in a hydrogen atmosphere to delete defects in the glass so as to improve the radiation resistance of the optical fiber. In the foregoing document, only a result of measurement of an increase in the loss experienced with respect to a near infrared rays having a wavelength of 1.3 .mu.m is described. In addition, the effect of improving the ultraviolet ray resistance obtained by the above-mentioned process disappears within about two months.
In "Improvement in Radiation Resistance of Optical Fiber by Hydrogen Treatment and .gamma.-Ray Irradiation", Tomon, Nagasawa, et al. pp. 1-213, Vol. 1, papers for lectures in National Conference of Semiconductor and Its Material Section of Electronic Communication Society, 1985, issued in 1985 by Electronic Communication Society (hereinafter referred to as "Document (3)"), a process has been reported for the purpose of preventing increase in light absorption of a pure silica-core optical fiber at a wavelength of 630 nm (visible ray) occurring due to .gamma.-ray irradiation. In this document, two-step treatment for an optical fiber is performed. In the first step, an optical fiber is doped with hydrogen molecules and then, in the second step, is irradiated with .gamma.-rays. Thus, seeds (precursors) of defects in the glass are converted into defects that absorb photon energy of a 2 eV band. Then, hydrogen previously dispersed in the fiber in the previous step and the defects in glass are chemically bonded to each other so as to improve the radiation resistance in the visible ray region. Also in the Document (3), there is no description about the characteristic of the fiber against ultraviolet rays.
U.S. Pat. No. 5,574,820 (hereinafter referred to as "Document (4)"), suggests an optical fiber and its manufacturing process that serves as a means for preventing increase in a loss in a visible ray region when a pure silica core fiber is used as an image fiber for transmitting visible rays in a radiation field. The proposed optical fiber is manufactured by previously irradiating pure silica core fiber with radiation in a large dose of 10.sup.5 Gy or greater, so that increase in the loss in a visible ray region having a wavelength of from 400 nm to 700 nm does not exceed 30 dB/km. Moreover, a process for manufacturing the optical fiber has been suggested, but the characteristic in the ultraviolet ray region has not been described.
JP-A-5-288942 (hereinafter referred to as "Document (5)"), as in Document (4), has suggested a process for improving radiation resistance of an image fiber for transmitting visible rays. In the process, an image fiber is irradiated with g-ryas in a large dose of 10.sup.7 Roentgen to 10.sup.9 Roentgen (10.sup.5 Gy to 10.sup.7 Gy) and then is heated in a hydrogen atmosphere. Also no description about the characteristic in the ultraviolet ray region has been made in the above-mentioned document.
In the Document (2), hydrogen is added so that the radioactive resistance of the optical fiber in the near infrared rays is improved. Recently there have been disclosed several processes in which hydrogen molecules are added to silica glass in an attempt to improve ultraviolet ray resistance. For example, JP-A-3-23236 (hereinafter referred to as "Document (6)") suggests silica glass in which OH groups are contained in an amount of 100 ppm or higher, substantially no oxygen defect exists and hydrogen gas is contained, so that ultraviolet ray resistance is improved. JP-A-5-32432 (hereinafter referred to as "Document (7)") suggests a process, in which deterioration due to ultraviolet ray irradiation is prevented by controlling the concentration of hydrogen in silica glass is to 1.5.times.10.sup.17 molecules/cm.sup.3 or higher. Moreover, the concentration of chlorine is made to be 100 ppm or lower to reduce hydrogen consumption in glass when ultraviolet ray irradiation is performed so as to maintain ultraviolet ray resistance. JP-A-6-16449 (hereinafter referred to as "Document (8)") suggests silica glass which has improved ultraviolet ray resistance by designing to contain OH group in an amount of 100 ppm or lower and chlorine in an amount of 200 ppm or lower, and to have a hydrogen concentration of 10.sup.16 molecules/cm.sup.3 or lower, a refractive index fluctuation of 5.times.10.sup.-6 or lower and a birefringence of 5 nm/cm or lower. U.S. Pat. No. 5,668,067 (hereinafter referred to as "Document (9)") suggests silica glass in which the amount of OH groups is 50 ppm or smaller and hydrogen is contained by at least 10.sup.18 molecules/cm.sup.3 and which is free from optical damage if the silica glass is irradiated with 10.sup.7 pulses of KrF laser, the output of which is 350 mJ/cm.sup.2. U.S. Pat. No. 5,679,125 (hereinafter referred to "Document (10)") suggests silica glass which has improved ultraviolet ray resistance because hydrogen molecules are added to silica glass to which fluorine has been added.
JP-A-7-300325 (hereinafter referred to as "Document (11)") suggests a process which is able to improve ultraviolet ray resistance by means similar to that suggested in Document (5) in such a manner that hydrogen-molecule-contained silica glass is irradiated with .gamma.-rays so as to make the concentration of hydrogen in the irradiated silica glass to be 5.times.10.sup.16 molecules/cm.sup.3 or higher so that the ultraviolet ray resistance is improved. JP-A-9-124337 (hereinafter referred to as "Document (12)") suggests a process with which the ultraviolet ray resistance is improved by irradiating glass containing hydrogen molecules at a concentration of from 2.times.10.sup.17 molecules/cm.sup.3 to 5.times.10.sup.19 molecules/cm.sup.3 with ultraviolet rays of 150 nm to 300 nm for 20 hours or longer.
The Document (1) discloses an optical fiber having an excellent initial transmission characteristic of ultraviolet rays. However, a satisfactory effect cannot be obtained to prevent the deterioration due to ultraviolet ray irradiation. On the contrary, absorption caused at the absorption edge of ultraviolet rays is enlarged undesirably. Therefore, adjustment of an optimum amount of addition cannot easily be achieved.
No description has been made about the deterioration due to ultraviolet ray irradiation in each of the Documents (2) to (5) relating to improvement in the radiation resistance required for transmitting near infrared rays. As described later, the processes adapted to a fiber for transmitting visible rays or a fiber for transmitting near infrared rays cannot maintain the effect to prevent deterioration due to ultraviolet ray irradiation for a required period of time. Moreover, unsuitable means for an optical fiber for transmitting ultraviolet rays have been employed.
The processes disclosed in the Documents (6) to (11) are arranged in such a manner that the contents of OH groups, F or Cl are adjusted. Although the above-mentioned adjustment of the components attains an effect of initial defects in glass, a satisfactory effect cannot be attained to reduce defects induced by ultraviolet rays.
The hydrogen treatment employed in the processes disclosed in Documents (6) to (12) is such that the defects in glass caused by ultraviolet ray irradiation and the hydrogen molecules dispersed in the glass by the hydrogen treatment are bonded to each other so that increase in absorption of light is restrained. The restraining period, however, is limited to a period of time in which hydrogen molecules remain in the glass. Since the processes disclosed in the Documents (6) to (12) are mainly adapted to a bulk-form glass member, the volume of the glass member is sufficiently large with respect to the velocity at which hydrogen in the glass is dispersed. It is considered that hydrogen molecules remain in the member for a long time and thus ultraviolet ray resistance can be maintained.
If the techniques in the Documents (6) to (12) are adapted to an optical fiber, hydrogen is undesirably dispersed out the outside in a short time. Thus, there arises a problem in that the ultraviolet ray resistance cannot be maintained. That is, hydrogen molecules in an optical fiber (having an outer diameter of 125 mm) are generally gradually discharged to the outside of the optical fiber at a room temperature and the concentration is lowered to about 1/10000 in about two months as shown in FIG. 6. That is, the above-mentioned restraining effect is effective in only about two months after the hydrogen treatment has been performed. Therefore, increase in the absorption cannot be restrained for a long time with the conventional techniques.